Illustrated Glossary of Organic Chemistry

Rate equation (rate law; rate expression): A mathematical description of a reaction's rate as a function of the concentration of the reactants, and the rate constant (k). Usually in a form similar to rate = k [reactant A] [reactant B].

In an S_N1 reaction, the rate-limiting step is ionization of the carbon-leaving group bond to produce a carbocation. This is a unimolecular process, for which the rate equation is: Rate = k [R₃C-LG].


In an S_N2 reaction, the rate-limiting step involves formation of the carbon-nucleophile bond simultaneously with ionization of the carbon-leaving group bond. This is a bimolecular process, for which the rate equation is: Rate = k [R₃C-LG] [Nuc].